The United States Government has rights to this invention pursuant to Contract Number N-66857-91-C-1034 awarded by Navy Ocean Systems Center, San Diego, Calif.
This invention relates generally to boron carbide/aluminum (B.sub.4 C/Al) cermets and their preparation. This invention relates more particularly to B.sub.4 C/Al cermets having improved properties through a controlled microstructure and their preparation.
U.S. Pat. No. 4,605,440 discloses a process for preparing B.sub.4 C/Al composites that includes a step of heating a powdered admixture of aluminum and boron carbide at a temperature of 1050.degree. C. to 1200.degree. C. The process yields, however, a mixture of several ceramic phases that differ from the starting materials. These phases, which include AlB.sub.2, Al.sub.4 BC, AlB.sub.12 C.sub.2, AlB.sub.12 and Al.sub.4 C.sub.3, adversely affect some mechanical properties of the resultant composite. In addition, it is very difficult to produce composites having a density greater than 99% of theoretical by this process. This may be due, in part, to reaction kinetics that lead to formation of the ceramic phases and interfere with the rearrangement needed to attain adequate shrinkage or densification. It may also be due, at least in part, to the lack of control over reactivity of molten aluminum. In fact, most of the aluminum is depleted due to formation of the reaction products.
U.S. Pat. No. 4,702,770 discloses a method of making a B.sub.4 C/Al composite. The method includes a preliminary step wherein particulate B.sub.4 C is heated in the presence of free carbon at temperatures ranging from 1800.degree. C. to 2250.degree. C. to provide a carbon enriched B.sub.4 C surface having a reactivity with molten aluminum that is lower than a B.sub.4 C surface without carbon enrichment. The reduced reactivity minimizes the undesirable ceramic phases formed by the process disclosed in U.S. Pat. No. 4,605,440. During heat treatment, the B.sub.4 C particles form a rigid network. The network, subsequent to infiltration by molten aluminum, substantially determines mechanical properties of the resultant composite. At temperatures in excess of 2000.degree. C., carbon distribution tends to be variable which leads, in turn, to different rates and degrees of sintering. The latter differences may result in cracking of parts having a thickness of 0.5 inch (1.3 cm) or greater.
U.S. Pat. No. 4,718,941 discloses a method of making metal-ceramic composites from ceramic precursor starting constituents. The constituents are chemically pretreated, formed into a porous precursor and then infiltrated with molten reactive metal. The chemical pretreatment alters the surface chemistry of the starting constituents and enhances infiltration by the molten metal. Ceramic precursor grains, such as boron carbide particles, that are held together by multiphase reaction products formed during infiltration form a rigid network that substantially determines mechanical properties of the resultant composite. A potential shortcoming of this method is that one cannot control the amount and size of phases that make up the multiphase reaction products.